1. Field of the Invention
This invention relates to racing engines and, in particular, to a multi-layer engine bearing having a relatively thick overlay for use in high speed and/or high load applications such as drag racing.
2. Disclosure of Related Art
In designing engine bearings for racing engines or other applications, several considerations must be taken into account. First, the bearings are designed for "conformability"--the ability of the bearing to conform and adapt to the position of the crankshaft in order to account for misalignment of the crankshaft and deflection of the crankshaft and other engine components under heavy loads. This property enables the load to be distributed across a relatively large surface area of the bearing. Second, the bearings are designed for "embedability"--the ability of the bearing surface to absorb particles found within the lubricant that flows between the crankshaft and bearing surfaces. This property prevents excess heat and friction from developing, prevents scoring of the crankshaft and bearing surfaces by the particles, and reduces the likelihood of seizure. Finally, the bearings are also designed to resist "fatigue" (i.e., fracturing of the bearing surface). Resistance to fatigue is dependent upon the bearing having a sufficient tensile strength to enable the configuration of the bearing surface to change without fracturing.
The engine bearings found in conventional drag racing engines are subject to extreme conditions. Conventional drag racing engines are supercharged and fueled by nitro-methane and alcohol. The engines operate at very high speeds (about 8,000-10,000 RPM), produce a very high horsepower output (about 8500 HP) and frequently detonate. As a result, the engines are subject to very high loads and the engines experience relatively severe deflection of the engine block, crankshaft, and connecting rods. This deflection creates misalignment between the surfaces of the crankshaft and the engine bearings (including the main bearings in the engine block and the connecting rod bearings). As a result, the outer edges of the bearings become overloaded. Further, the protective oil film between the crankshaft and bearing surfaces breaks down, thereby allowing the crankshaft and bearings to come into contact. The resulting friction and heat often leads to bearing failures.
A variety of bearings have been designed in an attempt to adapt to the extreme conditions found in conventional drag racing engines. One conventional bearing includes a backing layer comprised of steel and a layer of a babbitt metal. The babbitt metals are relatively soft thereby enhancing the conformability and embedability of the bearings. The babbitt metals are also relatively weak, however, and subject to fatigue. The babbitt metals tend to brake up and delaminate from the steel backing layer. The babbitt metals also wear rapidly, thereby allowing the crankshaft to come into contact with the steel backing layer. Finally, the babbitt metals extrude easily under high engine loads.
Another type of bearing found in conventional drag racing engines is the trimetal bearing. A trimetal bearing includes a backing layer comprised of steel, an intermediate layer of a strong bearing material for fatigue strength, and an overlay layer of a soft bearing material (e.g., lead(Pb)-tin(Sn)-copper(Cu)) for conformability and embedability. It is known in the art that improvements in conformability and embedability may be obtained by making the overlay layer thicker. It is also known in the art that improvements in fatigue strength may be obtained by making the overlay layer thinner. In a normal engine (i.e., one that is not subject to the loads imposed on racing engines), the overlay layer is generally about 0.001 inches (0.0254 mm) thick. Bearings having overlays thicker than 0.001 inches (0.0254 mm) are generally restricted to use in engines subject to only relatively light loads (approximately 2800 lbs/in.sup.2 (197 kg/cm.sup.2) maximum bearing load) where fatigue strength is not a problem. In racing engines, it is generally accepted in the art that the thickness of the overlay layer should be reduced from the normal 0.001 inches (0.0254 mm)--generally to about 0.0005 inches (0.0127 mm)--to improve fatigue strength and the service life of the bearings in the face of the extreme conditions presented in racing engines. This action, however, has reduced the conformability and embedability of the bearings. As a result, assembly and tolerances within the engine must be closely controlled to provide for successful operation of the engine.
There is thus a need for an improved racing engine that will minimize or eliminate one or more of the above-mentioned